1. Field of the Invention
This invention relates to an inkjet ink.
2. Description of the Related Art
In a situation where a fairly large number of copies of printed matter are required such as in the cases of local advertisement bill, enterprise handout and a postern of large scale, a printing machine utilizing a form plate has been conventionally employed for meeting such requirements. In recent years however, an on-demand printer which is capable of quickly coping with the diversification of needs and of minimizing stocks has been increasingly utilized in place of the aforementioned conventional printing machine. As for such an on-demand printer, an electrophotographic printer where a toner or a liquid toner is employed as well as an inkjet printer which is capable of achieving high-velocity and high-quality printing are expected to be useful.
As in the case of the printing machine utilizing a form plate, the on-demand printer employs a solvent type ink or a solvent type liquid toner both containing a pigment and an organic solvent. This technique of employing a solvent type ink or a solvent type liquid toner is however accompanied with the problem that the organic solvent volatilizes to such an extent that cannot be disregarded when the printing is repeated a certain number of times. Therefore, when this technique is employed, a problem of environmental contamination due to this volatilized organic solvent will be raised, thus necessitating the provision of an exhaust gas processing installation or a solvent recovery system.
On the other hand, in the case of the inkjet printer, since a solvent type ink can be handled in a closed system until the ink is discharged onto the surface of printing matter, it is possible, through the provision of suitable countermeasure for the exhaustion of solvent, to alleviate the aforementioned problem of environmental contamination. However, in different from the ink to be employed in the printing machine utilizing a form plate, the ink to be employed in the inkjet printer is required to have a suitable degree of fluidity for the delivery of ink. Therefore, the ink to be employed in the inkjet printer is required to be sufficiently high in the concentration of solvent, so that even with this technique, it is essentially difficult to overcome the problem of environmental contamination associated with the employment of the organic solvent.
Further, in the case of using a solvent type ink, the quality of printing is greatly influenced by the features of surface of printing matter. For example, if the surface of printing mater is permeable, bleeding is more likely to occur, and if the surface of printing mater is impermeable, it will become difficult to fix printed images. Moreover, since it will take a certain period of time in order to dry completely an ink layer that has been formed on the surface of printing matter, when an image is formed relatively thick on a large surface of printing matter, the image is more likely to collapse due to the fluidity of ink. Moreover, when the solvent type ink is employed, there is high possibility that the printed surface is deteriorated in the drying process of the ink layer. Thus, according to the aforementioned conventional techniques, it is not necessarily easy to obtain a printer matter with high qualities.
Incidentally, a photosensitive ink and a printer system employing the photosensitive ink are now taken notice of as a technique for coping with the aforementioned problem of solvent. The photosensitive ink to be employed this case typically comprises a radical polymeric monomer, a photopolymerization initiator and a pigment. Once this photosensitive ink is delivered onto a printing surface, it can be quickly photo-cured.
According to this technique, since the ink layer that has been formed on a printing surface can be non-fluidized by the irradiation of light, it is possible to obtain a printed matter which is relatively excellent in quality. However, this photosensitive ink includes a large amount of carcinogenic radical-generating agents such as a radical generating agent and still more, a volatile acrylic acid derivative to be employed as a radical polymeric monomer is highly stimulating to skin and badly smelling. Namely, it is necessary to take care in handling a photosensitive ink of this kind. Further, the radical polymerization is considerably obstructed by the presence of oxygen in air atmosphere and additionally, the light for exposure will be lost due to the absorption of light by the pigment included in the ink. As a result, underexposure may occur at a deep region of ink layer, so that the conventional radical polymeric ink is poor in sensitivity to light. Accordingly, in order to obtain a printed matter of high quality by using this conventional technique, it would become necessary to construct a large scale exposure system.
There has been proposed, as an ink which is relatively free from the influence of oxygen, a photosensitive ink which is cation photo-polymerizable. However, since the conventional ink of this type contains a solvent, the problem of the release of the solvent into environment cannot be overcome. Moreover, a cured layer is more likely to be further hardened, thus giving rise to the problem of clogging of the delivery head since this hardened layer is insoluble. Furthermore, there has been proposed a cation-cure type photosensitive composition which is adapted for inkjet delivery and can be employed for the coating of CD-ROM. The composition which has been found capable of performing the inkjet delivery mainly comprises vinyl ether and bisphenol A type epoxy, and hence there is still a serious problem associated with the release of these compounds to the environment.
There has been also proposed an inkjet ink having a specific composition including a cation-curable monomer. Since this ink also contains, as an essential component, a specific kind of vinyl ether compound which is high in volatility, there is the same problem as described above. Further, ordinary vinyl ether compounds are accompanied with the problem that when it is employed in combination with pigment, they become poor in polymerizability.
Additionally, there has been disclosed a photosensitive inkjet ink comprising a polyvalent vinyl ether compound and an alicyclic epoxy compound. The vinyl ethers disclosed are accompanied with various problems that they are poor in polymerizability, that the skeleton of bisphenol A exhibits its specific carcinogenic property, and that since the vinyl ethers are employed in combination with an alicyclic epoxy which is high in solubility and dark-reactivity, they are poor in shelf life and a printed matter to be obtained is not sufficiently resistive to a solvent.
In particular, when the printing surface is constituted by an absorptive medium, there will be raised the problem that the conventional acrylic photo-curable inkjet ink cannot be easily cured in the interior of the absorptive printing surface.
Incidentally, the conventional photo-cation-cure type inkjet ink is accompanied with the problem that the viscosity thereof can be greatly fluctuated. The reason for this can be attributed to the fact that once an acid generates due to changes with time of ink, the acid cannot be easily deactivated, thus giving rise to a great magnitude of dark reaction of ink. In the case of inkjet ink, when the viscosity thereof is caused to change, it will lead to the disturbance of the ejecting shape of ink, to poor reproducibility of printing, and in the worst case, to a critical condition such as defective delivery of ink and clogging of ink. Therefore, this problem is very serious.